Native messaging

This MR adds support for opening a native messaging port with the browser application to which the application has been installed. The practical use case is that this allows the Ceno Browser to request information from the extension that is only available to the extension, e.g. the source of content based on fields in the HTTP headers. Right now, we are only using this to report the count of sources for use in the Ceno Browser android app, but more use cases could be implemented, such as allowing the extension to notify the app of a failure to fetch content.

It should also be noted that while native messaging is technically not supported by Firefox for Android, it is still possible to use it with built-in extensions, which the Ceno extension does qualify as in the case of Ceno for Android.